Use flex layout instead of data-equalizer in feeds

Using data-equalizer is always hard due to the JavaScript it uses, while
the flex layout works all the time.

app/assets/stylesheets/widgets/feeds/participation.scss

@@ -1,20 +1,30 @@
 .feeds-participation {
 
+  @include breakpoint(medium) {
+    display: flex;
+
+    .feed-proposals:not(:only-child) {
+      width: 2 * 100% / 3;
+    }
+
+    .feed-debates:not(:only-child) {
+      width: 1 * 100% / 3;
+    }
+
+    .feed-proposals,
+    .feed-debates {
+      display: flex;
+      flex-direction: column;
+
+      .feed-content {
+        flex: 1;
+      }
+    }
+  }
+
   .feed-debates,
   .feed-proposals {
     @include grid-col;
     margin-top: $line-height;
   }
-
-  .feed-proposals:not(:only-child) {
-    @include breakpoint(medium) {
-      width: 2 * 100% / 3;
-    }
-  }
-
-  .feed-debates:not(:only-child) {
-    @include breakpoint(medium) {
-      width: 1 * 100% / 3;
-    }
-  }
 }